1. Field of Invention
This invention relates to high Q mechanical oscillators, and more particularly to mechanical oscillators suitable for use in high reliability sensors.
2. Description of the Prior Art
As known, oscillators having mechanical resonators, such as quartz crystals, tuning forks, balanced beams and vibrating cylinders, oscillate at a precision frequency in the range of resonant frequencies of the device as determined by the resonator material composition, structural dimensions and loading on the device. The mechanical oscillators have a higher Q than electronic oscillators which cause the displacement amplitude and frequency of oscillation to be particularly sensitive to damping effects caused both by internal friction and by loading of the resonator by external stimuli such as air resistance, temperature and pressure. The connection of the mechanical resonator to a resonator driver in a regenerative signal loop, whereby the driver provides the mechanical resonator with a sustaining force which overcomes the damping effects and maintains the displacement amplitude at a minimum measurable value while allowing the frequency to change in response to the selected external stimuli, results in a highly sensitive device for use in sensing selected parameters such as temperature and pressure. The drivers are electronic signal amplifiers which are coupled to the mechanical resonator through suitable pick-up and drive transducers. The transducers also provide an output resonant frequency signal representative of the mechanical oscillation frequency, which is digitally compatible such that the signal frequency is readily detected by a digital signal processor without analog-to-digital (A/D) interface, all of which is known.
The mechanical resonators themselves are inherently rugged and, therefore, reliable for use in sensors mounted in high vibration environments, such as the engine mounted sensors of an engine control system. The overall reliability of the sensors, however, is limited by the electronic resonator driver which is susceptible to failure from shock and vibration. Practical considerations of size and weight preclude the use of completely redundant sensors, such that sensor reliability has been improved in the prior art through use of redundant resonator drivers, where a first or primary driver circuit is operable at all times to provide the drive signal and a secondary driver is made operable only in the event of a failure of the primary. Only one driver is connected in the regenerative loop at any one time, requiring the use of electronic "enable/disable" switches to connect the secondary circuit in line. This results in a quasi-redundant sensor which can tolerate a single failure in the electronic components, i.e. the drivers or the switches.
The disadvantages of the prior art quasi-redundant design from the standpoint of reliability include: (1) the fact that electronic enable/disable switches degrade the effect of redundant drivers because their failure rates are usually significant when compared to the total failure rate of a single driver circuit, (2) the secondary driver circuit is non-operating during operation of the primary such that a failure in the secondary remains undetected, unless discovered in a "pre-use" checkout, until the primary driver fails, and (3) the absolute interruption of the resonant frequency signal in the interval of time required to sense the primary driver failure and switch over to the secondary driver followed by a transient settling time due to offset errors between the drivers, all of which results in a signal "down time" and loss of the control function for the interval. The inability to ascertain the health of the secondary driver during primary driver operation results in a loss of reliability "coverage," a critical reliability parameter. However, the most significant disadvantage may be the signal interruption due to switching which results in the momentary loss of control since interruption of the actual pressure signal at the discharge of the fan turbine stage for less than one second could result in engine runaway and destruction.